This application claims priority to Norwegian patent application Ser. No. 1999.1470, filed Mar. 25, 1999 and Norwegian patent application Ser. No. 2000.0831, filed Feb. 18, 2000. This application concerns a frame for stabilizing risers on a petroleum production vessel, preferably for production risers with xe2x80x9cdryxe2x80x9d wellheads, i.e. with Christmas trees arranged on the deck of a freely floating platform. The petroleum production considered takes place at very large sea depths, very likely more than 1200-1600 meters.
The invention is in one embodiment adapted for use in sea areas with the estimated wave height Hmax being in the order of amplitudes between 5 and 10 m, thus considerably less than the Hmax in the order of 25-30 m required for areas in the North Sea, conditions requiring considerably larger dimensioned and consequently heavier, more expensive vessels. A considerable problem in petroleum production at sea is to guide risers through the splash zone and the upper current- and wave-affected zone below the sea surface. In this zone, large tensions, tension variations, bending moments, wave actions and accelerations occur on the risers and their connection points, for example Christmas trees.
The prior art is described in the patent specifications GB 2 147 549, U.S. Pat. No. 5,558,467, and WO 95/28316. A similar shallow water construction which is not a vessel, and which cannot be applied in deep water, is described in GB 2 139 570.
xe2x80x9cSparxe2x80x9d Buoy
A deep semisubmersible construction called a xe2x80x9cSparxe2x80x9d buoy, may be adapted for production drilling, petroleum production or storing of petroleum fluids at sea. Such a design can consist of one single, heavily ballasted, column of very deep draught, having a relatively large buoyancy volume arranged at a high level in the column, at or below the water surface, and having a column through the splash zone and a work deck above water. The lower ballasted part can comprise a framework. Such a column stabilized construction design has little heave or vertical movement but its large draught can entail that even small angular movements, as measured in degrees, still entails considerable horizontal accelerations near the top and the lower end of the construction. Such a deep column-stabilized design has an advantage in that it encloses the risers in the critical area from the splash zone at the sea surface and down to a depth more than 100 meters so that wave and current forces do not reach the upper part of the risers. Such a design has economic disadvantages in that the deep draught requires heavier plate dimensioning to resist the higher water pressure. Heavier dimensioned steel plates entails higher weight and price. The deep draught of the assembled operative platform requires assembly at deep draught in deep water near the field, meaning higher lifting and assembly costs.
WO 99/10230 xe2x80x9cBuoyant substructure for offshore platformxe2x80x9d describes a buoyant substructure floating vertically standing in the sea (e.g. as an offshore platform) comprising at least three separate columns being interconnected. At least one of the columns is arranged to be ballasted by the end which is arranged to have deep draught, where the columns are interconnected by short beams.
NO 174 920 xe2x80x9cFlexible marine platform with surface production wells is described as a platform consisting of a rigid construction carrying a deck, pontoons fixed to the lower part of the rigid construction and a flexible construction constituted by columns fixed by their upper ends to the rigid construction and to the pontoons, and by their lower ends to a foundation arranged at the seabed, whereby the columns are in tension. Guide plates are illustrated, but no buoyancy elements on the risers.
Tension Leg Platforms
Another solution for production platforms is tension leg platforms, so-called TLP""s. Tension leg platforms are anchored via vertical tension legs or tethers anchored to the sea bed. The risers of such a tension leg platform may be guided by guide plates described in PCT publication WO 97/29944 and published Norwegian patent application NO 1998.3337, so that the risers get a parallel and small relative vertical movement relative to the platform deck and relative to each other. The tension legs are usually anchored with pile suction anchors, being vacuum-sucked down into the sediments in the sea bed, or gravity based structures on the seabed. At least two problems occur with such an anchoring solution:
a) At the large depths which may be in question: more than 1200-1600 meters, the seabed sediments may consist of less compacted unconsolidated organic mud, fine silt and clay particles with low density, low shear resistance and high water content, as distinct from glacially worked compacted clay/sand-containing sediments which constitute an essential part of the sea bed in the North Sea and the Norwegian Sea.
b) The sea bed can contain petroleum fractions forming so-called xe2x80x9chydratesxe2x80x9d being kept in a partial frozen phase at shallow depths below the sea bed and is presumed to be deposited from escaping petroleum fluids from deeper geological layers at higher temperatures. These hydrates are unstable and can pass to the gas/liquid phase if they are supplied with heat. In a sedimentary basin, deeper geological layers usually have a higher temperature than the surface layers. Petroleum production entails a heat transfer from the upwardly flowing/rising petroleum fluids in top of the well, to and may result in an unwanted fluidization of hydrates in layers close to the seabed. Thus, there is a risk of gas formation at the suction anchors and a risk for sudden loss of tension in a tension leg.
In deeper waters the separation between the risers must be large in order to avoid collision during hydrodynamic drag. This separation usually requires a larger and thus heavier tension leg platform.
Semisubmersible Platform
A third solution is ordinary column stabilized or semisubmersible constructions in the form of platforms. An essential problem with semisubmersible constructions with an open moonpool is that the production risers will hang freely movable and unstabilized through the splash zone and the upper water masses. This is difficult if there are buoyancy members on the risers, and especially if the buoyancy members are arranged in the splash zone, because, as mentioned above, problems with wave forces laterally and vertically on the buoyancy members and the risers occur.
A solution to the above mentioned problems is given according to one embodiment of the invention as defined in the patent claims enclosed: A system for use in petroleum production at sea, includes a guide frame for one or more riser pipes on a semisubmersible production vessel with one or more main buoyancy member arranged separately on at least one riser to carry the main part of the riser""s weight. Each riser is arranged for separately carrying a Christmas tree on top, near the deck of the vessel. The guide frame comprises vertical main elements arranged to extend vertically downwards from the deck, through the splash zone and through the upper, more wave- and current-influenced zone of the sea, down to a depth of about 50-150 meters below the sea surface, where drag forces are less pronounced. The novel features by the invention is as follows:
The guide frame has horizontal guide plates comprising vertically open cells formed of a horizontally arranged framework, preferably of beams, with lateral stabilization devices for guiding the risers"" and the main buoyancy members"" vertical relative movement and restricting the horizontal relative movement with respect to the guide frame.
The guide plates are arranged in at least two levels on the guide frame: (1) a lower guide plate is arranged at the lower ends of the vertical main elements of the guide frame, and (2) another guide plate is arranged at or just below the splash zone.
One main buoyancy member is arranged for being held on the riser preferably in level with, and guided by the lateral stabilization devices arranged in the lower guide plate, below the upper, more wave- and current-influenced zone near the sea surface, and with the risers being essentially without buoyancy elements through the splash zone, for being less exposed to the environmental water forces, e.g. drag, in the upper zone of the sea.
More specifically, the invention concerns a framework for arrangement in a production moonpool in a semisubmersible platform. In a preferred embodiment the semisubmersible platform has a square-shaped ring pontoon. The main proportion of wave-influence and current, exerting horizontal drag forces on risers, takes place in the upper 150 meters below the sea surface. These horizontal forces normally decrease strongly with increasing depth below the surface. According to a preferred embodiment of the invention, each riser is arranged with main buoyancy members or, so-called xe2x80x9ccansxe2x80x9d, arranged below the splash zone and the most strongly wave-influenced zone, so that they carry the main part of the weight of the riser in the sea. Thus the essential part of the carrying capacity represented by the wide-diameter buoyancy members is arranged in a depth zone where weaker drag forces are exerted. The diameter of auxiliary buoyancy elements is smaller further up, in order not to be so strongly affected by the more strongly wave- and current influencing zone of the water. Preferably the riser pipe is xe2x80x9cnakedxe2x80x9d through the splash zone, giving minimum attack surface for waves and current. The auxiliary buoyancy members may be arranged further upwards on the riser, to carry the local weight of the risers above the main buoyancy members. Below the buoyancy members, the risers are in tension. At a certain level between the buoyancy members and the wellhead on top of the riser, the longitudinal forces in the riser pipe pass from tension to compression. Thus each riser pipe according to the invention will carry a wellhead on top, being vertically arranged for free vertical movement relative to the platform deck. By reducing the exposed diameter of equipment crossing the splash zone, drag forces incurred by water currents and waves that pull or bend the riser laterally are minimized. The framework according to the invention is arranged to stiffen up the risers laterally in the current- and wave-influenced zone. In a preferred embodiment of the invention the framework extends to a depth of about 70 to 80 meters. The framework comprises, in a preferred embodiment of the invention, several levels with grid-like horizontal frames with one opening for each riser, with the opening preferably also containing a main or auxiliary buoyancy member. In the preferred embodiment, all openings for risers are sufficiently wide in order for the main buoyancy members to be set down through the framework right from the top. The largest buoyancy members are arranged in the deeper, less wave-influenced parts of the framework. This invention differs in this manner essentially from a so-called xe2x80x9cSparxe2x80x9d-buoy having the main buoyancy members arranged in or near the splash zone, protected through the splash zone by the surrounding cylindrical wall constituted by the column of the buoy.